-- be shared with us -- -- of their viral rules rule number one. Have a heart one of our biggest post was the most powerful images of 2011. And a lot of them are very serious moments the 9/11 memorial opening. The death of Steve Jobs tsunami in Japan and allow people to -- live and feel the pain in the poignancy that they had or that all year -- she's. Captures the moment little over a year ago there was this earthquake in New York City. And it was a small earthquake. But because we're not used to earthquakes here in New York we quickly get a post that was the most. Shocking pictures of the damage and it was like a yogurt cup knocked over her and after that -- this of that particular moment which is. A minor earthquake in your city that -- never going to lose this rule number three. Cute animals deserve respect there -- millions of people posting pictures of animals -- is -- much more competitive states of those others. Rule number four nostalgia sells. When you see a post about boy bands from the ninety's -- you see a post about candy can't eat anymore because they don't they stop making it and you remembered from the time your kid. It it's a powerful thing to connect -- other people who -- your age who have the same memories.

This transcript has been automatically generated and may not be 100% accurate.

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